Process for preparing bound zeolites

ABSTRACT

A process is described for the preparation of zeolitic catalysts in the form of microspheres, comprising zeolite and oligomeric silica, characterized by a high mechanical resistance. The process consists in subjecting to rapid drying the suspension, to which tetra-alkylorthosilicate is optionally added, resulting from the synthesis of zeolite by hydrothermal treatment at autogenous pressure of the reagent mixture containing tetra-alkylammoniumhydroxide as templating agent, and subjecting the product resulting from the drying to calcination.

[0001] The present invention relates to a new process for preparingzeolitic catalysts in the form of microspheres, comprising zeolite andoligomeric silica, characterized by a high mechanical resistance.

[0002] It is known that zeolites, synthetic porous crystalline materialshaving a catalytic activity, can be bound by means of inorganic bindersto increase the dimensions and enable them to be used in industrialplants. In fact, whereas on the one hand small dimensions of the zeolitecrystallites favour the intraparticle diffusion of the reagents andreaction products and allow good catalytic performances to be obtained,similar dimensions on the other hand make interparticle diffusionimpossible in fixed-bed reactors or the separation of the zeoliticcatalyst from the reaction medium in mixing reactors. To overcome this,the zeolites are bound with suitable binders. The preparation methods ofbound zeolites must be such as to prevent the blocking of the zeoliticcavities which would obviously cause a reduction in the catalyticactivity. For reactions in which a catalytic contribution on the part ofthe binder should be avoided, such as for example many oxidations andacid-catalyzed reactions, the use of a catalytically inert material suchas silica as binder is of particular interest. Silica however has poorbinding properties and is therefore not normally used in extrusion as itis not capable of providing the extruded product with sufficienthardness. EP 265018 describes a process for preparing catalysts based onzeolites and oligomeric silica. The zeolitic catalysts bound inmicrospheric form according to this method are characterized by a highmechanical resistance and are very suitable for reactions which takeplace in a fluid bed or in slurry reactors. This process comprises atleast the following steps:

[0003] a) preparation of the zeolite to be bound by crystallizationunder hydrothermal conditions of the appropriate mixture of reagents;

[0004] b) separation of the zeolite crystals obtained in phase (a);

[0005] c) possible washing of the zeolite by redispersion in water andnew separation of the crystalline phase;

[0006]2.

[0007] d) preparation of an aqueous solution of oligomeric silica andtetra-alkylammonium hydroxide by hydrolysis in liquid phase of atetra-alkylorthosilicate in an aqueous solution of tetra-alkylammoniumhydroxide, at a temperature ranging from 2 to 120° C., for a timeranging from 0.2 to 24 hours;

[0008] e) dispersion of the crystalline zeolitic phase obtained underpoint (c) in the solution described under point (d);

[0009] f) rapid drying of this dispersion by feeding to a spray-drier;

[0010] g) calcination of the microspheres of the product obtained in thedrying step.

[0011] We have now found a considerably simplified process for preparingcatalysts comprising zeolites and oligomeric silica having a highmechanical resistance. This simplified process is capable of giving thebinding phase particular properties such as pore distributionessentially in the mesoporous region and a high surface area, whichguarantee the absence of diffusional barriers on the part of the binderwhile the catalytic properties of the zeolite remain unaltered. This newprocess at the same time allows both a reduction in the number ofunitary operations required and also a reduction in the quantity ofreagents used. In particular, unlike the prior art, the process of thepresent invention no longer requires a separation passage of thezeolitic crystalline phase from the suspension obtained at the end ofthe synthesis: it has in fact been unexpectedly found that, for zeoliteswhose synthesis require the presence of tetra-alkylamonium hydroxide(TAAOH) as templating agent, i.e. as a compound capable of inducing aregular and constant porosity in the structure of the zeolite, theresulting suspension at the end of the synthesis, in which zeolitecrystals and tetra-alkylammoniumhydroxide remaining in solution, arepresent, can be used as such, without further purification and/orfiltration passages, for the preparation of zeolites bound witholigomeric silica in the form of microspheres. The possibility ofavoiding the separation step of the zeolite is, from an industrial pointof view, a great advantage, particularly when operating with zeolitecrystals having dimensions of less than 0.5μ, which cannot be separatedfrom the synthesis medium with the usual techniques, for example byfiltration or continuous centrifuges, but require the use of more costlytechniques operating batchwise.

[0012] The present invention therefore relates to a process for thepreparation of zeolitic catalysts in the form of microspheres comprisingzeolite and oligomeric silica which consists in subjecting to rapiddrying the suspension, to which tetra-alkylorthosilicate is optionallyadded, resulting from the synthesis of zeolite by hydrothermal treatmentat autogenous pressure of the reagent mixture containingtetra-alkylammonium hydroxide as templating agent, and subjecting theproduct resulting from the drying to calcination.

[0013] In the suspension obtained at the end of the synthesis of thezeolite by hydrothermal treatment in the presence of tetra-alkylammoniumhydroxide (TAAOH) as templating agent, which is subjected to rapiddrying according to the present invention, zeolite crystals, thefraction of tetra-alkylammonium hydroxide which has not been englobedinside the pores of the zeolite and possibly oxides of silicon and otherheteroatoms, will be present. It is in fact known that in the synthesisof zeolites, it is necessary to use an excess of tetra-alkylammoniumhydroxide and consequently part of the TAAOH is in solution in thereaction medium at the end of the synthesis. The use of this suspensionresulting from the synthesis of the zeolite, in addition to theadvantages described above, therefore enables a reduction in the totalquantity of TAAOH used in the process for the preparation of zeolitesbound with oligomeric silica, with respect to that necessary in theprocess of the prior art EP 265018 in which it was necessary to prepareseparately a mixture of TAAOH and oligomeric silica and add it to thezeolite crystals, separated by filtration or centrifugation, before therapid drying step.

[0014] When, according to the present invention,tetra-alkylorthosilicate is added to the suspension resulting from thesyntheis of zeolite, before this is subjected to rapid drying, it willbe added in a quantity ranging from 0.08 to 0.50 moles per 100 grams ofzeolite contained in the suspension. The tetra-alkylorthosilicate isselected from compounds having the formula Si(OR)₄ wherein the Rsubstituents, which can be equal to or different from each other, arealkyl chains containing from 1 to 4 carbon atoms.

[0015] The rapid drying of the suspension is preferably carried out byfeeding to a spray-drier. At the end of the rapid drying treatmentmicrospheres are obtained which are calcined at a temperature rangingfrom 400 to 800° C.

[0016] The zeolitic catalysts obtained according to the process of thepresent invention are characterized by a high mechanical resistance,they comprise oligomeric silica and zeolite in a weight ratio rangingfrom 0.05 to 0.30 and are in the form of microspheres having a diameterranging from 5 to 300 μm in which the zeolite crystals are encaged bySi—O—Si bridges. The zeolites which can be bound with oligomeric silicaby means of the process of the present invention are those whosepreparation comprises the use of tetra-alkylammonium hydroxide astemplating agent, in particular zeolites which can be well used can beselected from those with an MFI, MEL, MFI/MEL, MOR, FAU, FAU/EMT and BEAstructure.

[0017] Suspensions which can be subjected to rapid drying and subsequentcalcination to obtain zeolites bound with oligomeric silica, accordingto the present invention, are those resulting from the synthesis ofzeolite under hydrothermal conditions, at autogenous pressure, and inthe presence of TAAOH, effected with the methods described in the priorart and known to experts in the field: in particular, according to apreferred aspect of the present invention, the zeolite which is bound isthe silicalite belonging to the MFI group, or a zeolite consisting ofsilicon and aluminum oxides having an MFI, MEL, MFI/MEL, BEA, MOR, FAUand FAU/EMT structure. The suspensions resulting from these synthesescontain zeolite crystals, tetra-alkylammonium hydroxide, silicon andoptionally aluminum oxides; in this case the addition oftetra-alkylorthosilicate is not necessary; these suspensions are feddirectly to the spray-drier and the resulting microspheres calcinedunder the conditions described above.

[0018] One aspect of the present invention is therefore a process forthe preparation of zeolitic catalysts in the form of microspherescomprising zeolite and oligomeric silica, wherein the zeolite isselected from silicalite belonging to the MFI group, or a zeoliteconsisting of silicon and aluminum oxides having an MFI, MEL, MFI/MEL,BEA, MOR, FAU and FAU/EMT structure, which consists in subjecting torapid drying, the suspension resulting from the synthesis of saidzeolite by hydrothermal treatment at autogenous pressure of the reagentmixture containing tetra-alkylammonium hydroxide as templating agent,and subjecting the product resulting from the drying to calcination.

[0019] The conditions under which these suspensions are prepared arethose known to experts in the field and are normally described in theprior art. For example, the silicalite belonging to the MFI group,called S-1, and its preparation are described in U.S. Pat. No.4,061,724; a BEA zeolite consisting of silicon and aluminum oxides,called zeolite beta, and its preparation, are described in U.S. Pat. No.3.308.069; an MFI zeolite consisting of silicon and aluminum oxides,called ZSM-5, and its preparation are described in U.S. Pat. No.3,702,886 and in the reissue U.S. Pat. No. 29,948; a MOR zeoliteconsisting of silicon and aluminum oxides, called mordenite, and itspreparation are described in U.S. Pat. No. 4,052,472; a FAU zeoliteconsisting of silicon and aluminum oxides, called N-Y, and itspreparation are described in U.S. Pat. No. 3,306,922; a FAU/EMT zeoliteconsisting of silicon and aluminum oxides, called ECR-30 and itspreparation are described in EP 315461; an MFI/MEL zeolite consisting ofsilicon and aluminum oxides, called ZSM-5/ZSM-11 is described in U.S.Pat. No. 4,289,607; a MEL zeolite consisting of silicon and aluminumoxides, called ZSM-11 is described in U.S. Pat. No. 3,709,979.

[0020] The product resulting from the drying, before being calcined, canoptionally be subjected to a treatment in air at 200-300° C. for 2-10hours and an exchange in acid form according to the known techniques toremove the alkaline metals possibly present.

[0021] The most suitable zeolite for being bound according to thispreferred aspect is silicalite S-1. The zeolitic material inmicrospheres having a diameter of 5 to 300 μm, consisting of silicaliteS-1 and oligomeric silica, in a weight ratio oligomericsilica/silicalite ranging from 0.05 to 0.3, characterized by a highmechanical resistance, used in the field of catalysis, is new and is afurther object of the present invention. When zeolites consisting ofsilicon and aluminum oxides belonging to the MFI, MEL, MFI/MEL, BEA,MOR, FAU and FAU/EMT groups, are used, small quantities of alumina arealso present in the binding phase of the end-catalyst. These lattercatalysts prepared according to the process of the present invention,having a high mechanical resistance, which are useful in transformationprocesses of hydrocarbons, as acid catalysis reactions, are new and area further object of the present invention.

[0022] According to a particularly preferred aspect of the presentinvention, when the zeolite to be bound belongs to the MFI, MFI/MEL andMEL group, and optionally contains other heteroatoms as well as silicon,the suspension which is subjected to rapid drying is that deriving fromthe synthesis of the zeolite produced so as to obtain a crystallizationyield of the zeolite as close as possible to 100%, for example between98 and 100%. It is particularly preferable to operate in such a way asto obtain a crystallization yield of 100% which corresponds to a totalrecovery in the zeolite of all the silica and possible heteroatompresent in the reagent mixture. The zeolites of the MFI, MFI/MEL and MELgroup which are particularly suitable for being prepared with acrystallization yield of 100% and are therefore preferably boundaccording to this preferred aspect of the present invention, areselected from:

[0023] 1) MFI zeolites having the formula p HMO₂·q TiO₂·SiO₂, wherein Mis a metal selected from aluminum, gallium and iron, p has a valueranging from 0 to 0.04 and q has a value ranging from 0.0005 to 0.03. Inparticular when p is 0, the zeolite is titanium silicalite TS-1described in U.S. Pat. No. 4,410,501; zeolites in which p is differentfrom 0 and M=Al, Ga and Fe are described in EP 226257, EP 266825 and EP226258, respectively;

[0024] 2) MFI zeolites having the formula a Al₂O₃·(1−a)SiO₂, wherein ahas a value ranging from 0 to 0.02. In particular when a is 0 thezeolite is the silicalite S-1 described in U.S. Pat. No. 4,061,724; whena is different from 0 the zeolite is ZSM-5 described in U.S. Pat. No.3,702,886 and in the reissue U.S. Pat. No. 29,948;

[0025] 3) MEL or MFI/MEL zeolites having the formula x TiO₂·(1−x)SiO₂,wherein x has a value ranging from 0.0005 to 0.03. These zeolites aredescribed in BE 1001038 and are called TS-2 and TS-1/TS-2.

[0026] Therefore, in accordance with the preferred aspect of operatingso that the suspension subjected to rapid drying derives from thesynthesis of an MFI zeolite with a crystallization yield of more than98%, preferably 100%, the present invention relates to a process for thepreparation of zeolitic catalysts in the form of microspheres,consisting of MFI zeolite having the formula p HMO₂·q TiO₂·SiO₂ andoligomeric silica, wherein M is a metal selected from aluminum, galliumand iron, p has a value ranging from 0 to 0.04 and q has a value rangingfrom 0.0005 to 0.03, which consists in:

[0027] a) synthesis of the zeolite by means of hydrothermal treatment atautogenous pressure, at a temperature ranging from 190 to 230° C. andfor a time ranging from 0.5 to 10 hours, without alkaline metals, of amixture containing a silicon source, a titanium source, optionally asource of a metal M, and tetrapropylammoniumhydroxide, having thefollowing composition expressed as molar ratios:

[0028] Si/Ti=35-2000

[0029] M/Si=0-0.04 wherein M is selected from Al, Ga and Fe

[0030] TPA-OH/Si=0.2-0.5 wherein TPA=tetrapropylammonium

[0031] H₂O/Si=10-35

[0032] b) addition of tetra-alkylorthosilicate to the suspensionresulting from the previous step a);

[0033] c) rapid drying of the suspension obtained in step b);

[0034] d) calcination of the product obtained in step c).

[0035] The silicon, titanium and metal sources are those described inU.S. Pat. No. 4,410,501, EP 226257, EP 266825 and EP 226258. The siliconsource is preferably tetraethylorthosilicate, the titanium source ispreferably tetraethylorthotitanate, the metal source is preferably asoluble salt of the metal. The zeolite which is preferably boundaccording to this method is titanium-silicalite TS-1.

[0036] In accordance with the preferred aspect of operating so that thesuspension subjected to rapid drying derives from the synthesis of anMFI zeolite with a crystallization yield of more than 98%, preferably100%, and the MFI zeolite to be bound is a Al₂O₃·(1−a) SiO₂, a furtherobject of the present invention relates to a process for the preparationof zeolitic catalysts in the form of microspheres, consisting of MFIzeolite having the formula a Al₂O₃·(1−a) SiO₂ and oligomeric silica,wherein a has a value ranging from 0 to 0.02, which consists in:

[0037] a) synthesis of the zeolite by means of hydrothermal treatment atautogenous pressure, at a temperature ranging from 190 to 230° C. andfor a time ranging from 0.5 to 10 hours, without alkaline metals, of amixture containing a silicon source, optionally an aluminum source,tetrapropylammonium hydroxide, having the following compositionexpressed as molar ratios:

[0038] Al/Si=0-0.04

[0039] TPA-OH/Si=0.2-0.5 wherein TPA=tetrapropylammonium

[0040] H₂O/Si=10-35

[0041] b) addition of tetra-alkylorthosilicate to the suspensionresulting from the previous step a);

[0042] c) rapid drying of the suspension obtained in step b);

[0043] d) calcination of the product obtained in step c).

[0044] The silicon and aluminum sources are those described in U.S. Pat.No. 4,061,724 and U.S. Pat. No. 3,702,886. The silicon source ispreferably tetraethylorthosilicate, the aluminum source is preferablyAl(OR)₃ wherein R is an alkyl containing from 3 to 4 carbon atoms.

[0045] In accordance with the preferred aspect of operating so that thesuspension subjected to rapid drying derives from the synthesis of anMFI/MEL or MEL zeolite with a crystallization yield of more than 98%,preferably 100%, and the MFI/MEL or MEL zeolite to be bound is xTiO₂·(1−x) SiO₂, a further object of the present invention relates to aprocess for the preparation of zeolitic catalysts in the form ofmicrospheres, consisting of MFI/MEL or MEL zeolite having the formula xTiO₂·(1−x) SiO₂ and oligomeric silica, wherein x has a value rangingfrom 0.0005 to 0.03, which consists in:

[0046] a) synthesis of the zeolite by means of hydrothermal treatment atautogenous pressure, at a temperature ranging from 190 to 230° C. andfor a time ranging from 0.5 to 10 hours, without alkaline metals, of amixture containing a silicon source, a titanium source,tetra-alkylammoniumhydroxide, having the following composition expressedas molar ratios:

[0047] Si/Ti=35-2000

[0048] TAA-OH/Si=0.2-0.5

[0049] H₂O/Si=10-35

[0050] b) addition of tetra-alkylorthosilicate to the suspensionresulting from the previous step a);

[0051] c) rapid drying of the suspension obtained in step b);

[0052] d) calcination of the product obtained in step c).

[0053] The silicon source, the titanium source and thetetra-alkylammoniumhydroxide which can be used, also in binary orternary mixtures, are those described in BE 1.001.038. The siliconsource is preferably tetraethylorthosilicate, the titanium source ispreferably tetraethylorthotitanate.

[0054] The hydrothermal treatment in the previous synthesis steps (a) ofthe zeolite is preferably carried out at a temperature ranging from 200to 230° C., and is effected, for example, at a temperature higher than200° C. and less than or equal to 230° C.

[0055] The particular composition combination of the reaction mixtureand reaction temperature used in the previous steps a), which enablesthe preparation of zeolites with an MFI, MFI/MEL and also MEL structure,in particular TS-1 and S-1, with a very high crystallization yield,preferably 100%, is in itself new and is a further aspect of the presentinvention. In the examples relating to the preparation of zeolites withan MFI, MEL and MFI/MEL structure described in the prior art thecrystallization yield specified or calculable is also much lower than100%.

[0056] Using this particular composition combination and reactionconditions in step (a), microspheres are obtained, at the end of stepd), consisting of MFI, MEL, or MFI/MEL zeolite, bound with oligomericsilica, preferably TS-1 and S-1, with a weight ratio oligomericsilica/zeolite ranging from 0.05 to 0.3, having a high surface area, apore distribution in the binding phase essentially in the mesoporousregion, a high mechanical resistance, which can be used in the field ofcatalysis. In particular, the zeolitic material in microspheres having adiameter ranging from 5 to 300 Am, consisting of zeolite TS-2 orTS-1/TS-2 and oligomeric silica, in a weight ratio oligomericsilica/zeolite ranging from 0.05 to 0.3 is new and is a further objectof the present invention.

[0057] In the previous steps (b) the tetra-alkylorhtosilicate (TAOS),preferably tetraethylorthosilicate, is added in a quantity ranging from0.08 to 0.50 moles per 100 grams of zeolite contained in the suspensionobtained at the end of step a).

[0058] The suspension deriving from step (b) is preferably heated to40-100° C. for 0.5-10 hours, before being subjected to the rapid dryingstep.

[0059] In the previous steps (c), the suspension obtained from step (b)is subjected to rapid drying, preferably with a spray-drier, obtainingmicrospheres consisting of a tridimensional silica lattice in which thezeolite crystallites are tightly encaged with Si—O—Si bridges.

[0060] The microspheres resulting from step (c) are calcined at atemperature ranging from 400 to 800° C.

EXAMPLE 1

[0061] 1873 g of tetrapropylammonium hydroxide (TPA-OH) at 14% by weightin aqueous solution are charged into an autoclave. 547 g oftetraethylorthosilicate (TEOS) and a solution consisting of 547 g ofTEOS and 30 g of tetraethylorthocitanate (TEOT) are then rapidly addedin succession, operating in a closed system. Without subjecting themixture thus prepared to aging, the hydrothermal treatment isimmediately initiated at autogenous pressure at 200° C. for 2 hours.When the crystallization has terminated, the autoclave is cooled and alactescent suspension is discharged.

[0062] 100 g of this suspension are centrifuged, redispersed in waterand centrifuged again; the solid obtained is dried, calcined and used tocharacterize the crystalline phase. The crystallization yield proved tobe 100%, chemical analysis gave the following result: SiO₂=96.8%,TiO₂=3.19%

[0063] Upon TEM analysis, crystallines agglomerations are with anaverage diameter of 0.3 μm are observed.

[0064] The UV-Vis spectrum is shown in FIG. 1 curve A (the wave lengthis indicated in abscissa, the absorbance in ordinate).

[0065] 110 g of TEOS are added to the remaining lactescent suspension,the mixture is heated to 60° C. for 1 hour and then sent to thespray-drier (Niro Mobile Minor HI-TEC, temperature of in-going air 230°C.; temperature of out-going air 150° C., diameter of chamber: 1 m).Compact microspheres having an average diameter of 30 μm, are obtained,and a weight ratio oligomeric silica/zeolite of 0.1. The microspheresare put in muffle in a nitrogen atmosphere and heated to 550° C. After 2hours of residence at that temperature in nitrogen, the atmosphere isgradually changed from nitrogen to air and the product left for afurther 2 hours at 550° C. in air. The product obtained has thefollowing composition

SiO₂=97.05% TiO₂=2.94%

[0066] The UV-Vis spectrum is shown in FIG. 1, curve B. On comparing thespectra indicated in FIG. 1, it can be seen that in both the samples allthe titanium is in tetrahedric coordination and thus inserted in thezeolitic framework. FIG. 2 indicates the distribution of the dimensionsof the microspheres, determined by Granulometre 715 E608, measuredbefore the ultrasound treatment (curve

) and after treatment for 1 hour in ultrasounds (curve

) (Branson Bath 5200). The average diameter of the microspheresexpressed in μm is indicated in abscissa, the % of microspheres inordinate. From the figure it can be observed that the particledistribution is not modified after the ultrasound treatment and that thecatalyst therefore has a good mechanical resistance.

EXAMPLE 2

[0067] 2288 g of tetrapropylammonium hydroxide (TPA-OH) at 14% by weightin aqueous solution are dissolved in 7 g of aluminum isopropylate and1094 g of tetraethylorthosilicate (TEOS) are then added. The solutionobtained is charged into an autoclave and subjected to hydrothermaltreatment at autogenous pressure at 200° C. for 2 hours. When thecrystallization has terminated, the autoclave is cooled and a lactescentsuspension is discharged.

[0068] 100 g of this suspension are centrifuged, redispersed in waterand centrifuged again; the solid obtained is dried, calcined and used tocharacterize the crystalline phase. The crystallization yield proved tobe 100%, chemical analysis gave the following result:

SiO₂=99.44%, Al₂O₃=0.56%

[0069] 110 g of TEOS are added to the remaining lactescent suspension,the mixture is heated to 60° C. for 1 hour and then sent to thespray-drier (Niro Mobile Minor HI-TEC, temperature of in-going air 230°C.; temperature of out-going air 150° C., diameter of chamber: 1 m)Compact microspheres having an average diameter of 30 μm, are obtained,and a weight ratio oligomeric silica/zeolite of 0.1. The microspheresare put in muffle in a nitrogen atmosphere and heated to 550° C. After 2hours of residence at that temperature in nitrogen, the atmosphere isgradually changed from nitrogen to air and the product left for afurther 2 hours at 550° C. in air. The product obtained has thefollowing composition

SiO₂=99.49% Al₂O₃=0.51%

[0070]FIG. 3 shows the distribution of the dimensions of themicrospheres, determined by Granulometre 715 E608, measured before theultrasound treatment (curve

) and after treatment of 1 hour in ultrasounds (curve

) (Branson Bath 5200). The average diameter of the microspheresexpressed in μm is indicated in abscissa, the % of microspheres inordinate. From the figure it can be observed that the particledistribution is not modified after the ultrasound treatment and that thecatalyst therefore has a good mechanical resistance.

EXAMPLE 3

[0071] 1102 g of water and 1096 g of TEOS are added to 802 g of TPA-OHat 40%. The solution obtained is charged into an autoclave and subjectedto hydrothermal treatment for 3 hours at 180° C. The autoclave is thencooled and a lactescent suspension is discharged. 100 g of thissuspension are centrifuged, redispersed in water and centrifuged again;the solid obtained is dried, calcined. The crystallization yield provedto be 84%. The remaining suspension is sent as such to the spray-drier(Buchi 190, temperature at the inlet 200° C.; temperature at the outlet140° C.).

[0072]FIG. 4 shows the distribution of the dimensions of themicrospheres, determined by Granulometre 715 E608, measured before theultrasound treatment (curve

) and after treatment of 2 hours in ultrasounds (curve

) (Branson Bath 5200). The average diameter of the microspheresexpressed in μm is indicated in abscissa, the % of microspheres inordinate. From the figure it can be observed that the particledistribution is not modified after the ultrasound treatment and that thecatalyst therefore has a good mechanical resistance.

EXAMPLE 4 (COMPARATIVE)

[0073] A TS-1 is prepared in accordance with U.S. Pat. No. 4,410,501: asolution consisting of 2844 g of TEOS and 153 g of TEOT is added over aperiod of one hour to 4662 g of TPAOH at 15% by weight in aqueoussolution. The end solution obtained is heated slightly to accelerate thehydrolysis and evaporate the ethyl alcohol formed. After about 5 hoursat 80° C., 5850 g of water are added. The end solution is charged intoan autoclave and heated to 180° C. at autogenous pressure for 5 hours.When the crystallization has terminated, the autoclave is cooled and alactescent suspension is discharged. 1000 g of this suspension arecentrifuged, redispersed in water and centrifuged again; the solidobtained is dried, calcined and used to characterize the crystallinephase. The crystallization yield proved to be 89%, chemical analysisgave the following result:

SiO₂=93.97%, TiO₂=3.10%

1) a process for the preparation of zeolitic catalysts in the form ofmicrospheres, comprising zeolite and oligomeric silica, which consistsin subjecting to rapid drying the suspension to whichtetra-alkylorthosilicate is optionally added, resulting from thesynthesis of the zeolite by hydrothermal treatment at autogenouspressure of the reagent mixture containing tetra-alkylammonium hydroxideas templating agent, and subjecting the product resulting from thedrying to calcination. 2) The process according to claim 1, wherein thezeolite is selected from zeolites having an MFI, MFI/MEL, MEL, BEA, MOR,FAU and FAU/EMT structure. 3) The process according to claim 1, whereinthe tetra-alkylorthosilicate is added in a quantity ranging from 0.08 to0.50 moles per 100 grams of zeolite contained in the suspensionresulting from the synthesis. 4) The process according to claim 3,wherein the tetra-alkylorthosilicate is selected from the compoundshaving the formula Si(OR)₄ wherein the substituents R, the same ordifferent, are alkyl chains containing from 1 to 4 carbon atoms. 5) Theprocess according to claim 1, wherein the rapid drying is effected byfeeding to a spray-drier. 6) The process according to claim 1, whereinthe calcination is carried out at a temperature ranging from 400 to 800°C. 7) The process according to claim 1, which consists in subjecting torapid drying, the suspension resulting from the synthesis of the zeoliteby hydrothermal treatment at autogenous pressure of the reagent mixturecontaining tetra-alkylammoniumhydroxide as templating agent, andsubjecting the product resulting from the drying to calcination, whereinthe zeolite is selected from silicalite belonging to the MFI group or azeolite consisting of silicon and aluminum oxides having an MFI,MFI/MEL, MEL, BEA, MOR, FAU and FAU/EMT structure. 8) The processaccording to claim 1 for the preparation of zeolitic catalysts in theform of microspheres, consisting of MFI zeolite having the formula pHMO₂·q TiO₂·SiO₂ and oligomeric silica, wherein M is a metal selectedfrom aluminum, gallium and iron, p has a value ranging from 0 to 0.04and q has a value ranging from 0.0005 to 0.03, which consists in: a)synthesis of the zeolite by means of hydrothermal treatment atautogenous pressure, at a temperature ranging from 190 to 230° C. andfor a time ranging from 0.5 to 10 hours, without alkaline metals, of amixture containing a silicon source, a titanium source, optionally asource of a metal M, and tetra-propylammoniumhydroxide, having thefollowing composition expressed as molar ratios: Si/Ti=35-2000M/Si=0-0.04 wherein M is selected from Al, Ga and Fe TPA-OH/Si=0.2-0.5wherein TPA=tetrapropylammonium H₂O/Si =10-35 b) addition oftetra-alkylorthosilicate to the suspension resulting from the previousstep a); c) rapid drying of the suspension obtained in step b); d)calcination of the product obtained in step c). 9) The process accordingto claim 8, wherein the silicon source is tetraethylorthosilicate, thetitanium source is tetraethylorthotitanate and the metal source is asoluble salt of the metal. 10) The process according to claim 8, whereinthe zeolite is titanium-silicalite TS-1. 11) The process according toclaim 1 for the preparation of zeolitic catalysts in the form ofmicrospheres, consisting of MFI zeolite having the formula a Al₂O₃·(1−a)SiO₂ and oligomeric silica, wherein a has a value ranging from 0 to0.02, which consists in: a) synthesis of the zeolite by means ofhydrothermal treatment at autogenous pressure, at a temperature rangingfrom 190 to 230° C. and for a time ranging from 0.5 to 10 hours, withoutalkaline metals, of a mixture containing a silicon source, optionally analuminum source, tetrapropylammoniumhydroxide, having the followingcomposition expressed as molar ratios: Al/Si=0-0.04 TPA-OH/Si=0.2-0.5wherein TPA=tetrapropylammonium H₂O/Si=10-35 b) addition oftetra-alkylorthosilicate to the suspension resulting from the previousstep a); c) rapid drying of the suspension obtained in step b); d)calcination of the product obtained in step c). 12) The processaccording to claim 11, wherein the silicon source istetraethylorthosilicate and the aluminum source is Al(OR)₃ wherein R isan alkyl containing from 3 to 4 carbon atoms. 13) The process accordingto claim 11, wherein the zeolite is silicalite S-1. 14) The processaccording to claim 1 for the preparation of zeolitic catalysts in theform of microspheres, consisting of MFI/MEL or MEL zeolite having theformula x TiO₂·(1−x) SiO₂ and oligomeric silica, wherein x has a valueranging from 0.0005 to 0.03, which consists in: a) synthesis of thezeolite by means of hydrothermal treatment at autogenous pressure, at atemperature ranging from 190 to 230° C. and for a time ranging from 0.5to 10 hours, without alkaline metals, of a mixture containing a siliconsource, a titanium source, tetra-alkylammoniumhydroxide, having thefollowing composition expressed as molar ratios: Si/Ti=35-2000TAA-OH/Si=0.2-0.5 H₂O/Si=10-35 b) addition of tetra-alkylorthosilicateto the suspension resulting from the previous step a); c) rapid dryingof the suspension obtained in step b); d) calcination of the produceobtained in step c). 15) The process according to claim 14, wherein thesilicon source is tetraethylorthosilicate and the titanium source istetraethylorthotitanate. 16) The process according to claims 8, 11 and14, wherein the hydrothermal treatment in step a) is carried out at atemperature ranging from 200 to 230° C. 17) The process according toclaims 8, 11 and 14, wherein, in step b), the tetra-alkylorthosilicateis added in a quantity ranging from 0.08 to 0.50 moles per 100 grams ofzeolite contained in the suspension resulting from step a). 18) Theprocess according to claims 8, 11 and 14, wherein in step b) thetetra-alkylorthosilicate is tetraethylorthosilicate. 19) The processaccording to claims 8, 11 and 14, wherein the calcination in step c) iscarried out at a temperature ranging from 400 to 800° C. 20) A processfor preparing MFI zeolites having the formula p HMO₂·q TiO₂·SiO₂,wherein M is a metal selected from aluminum, gallium and iron, p has avalue ranging from 0 to 0.04 and q has a value ranging from 0.0005 to0.03, which consists in subjecting to hydrothermal treatment atautogenous pressure, at a temperature ranging from 190 to 230° C. andfor a time ranging from 0.5 to 10 hours, without alkaline metals, amixture containing a silicon source, a titanium source, optionally asource of a metal M, and tetrapropylammoniumhydroxide, having thefollowing composition expressed as molar ratios: Si/Ti=35-2000M/Si=0-0.04 wherein M is selected from Al, Ga and Fe TPA-OH/Si=0.2-0.5wherein TPA=tetrapropylammonium H₂O/Si 10-35 21) The process accordingto claim 20 wherein the zeolite is TS-1. 22) A process for preparingzeolites of the MFI group having the formula a Al₂O₃·(1−a) SiO₂, whereina has a value ranging from 0 to 0.02, which consists in subjecting tohydrothermal treatment at autogenous pressure, at a temperature rangingfrom 190 to 230° C. and for a time ranging from 0.5 to 10 hours, withoutalkaline metals, a mixture containing a silicon source, optionally analuminum source and tetrapropylammoniumhydroxide, having the followingcomposition expressed as molar ratios: Al/Si=0-0.04 TPA-OH/Si=0.2-0.5wherein TPA=tetrapropylammonium H₂O/Si=10-35. 23) The process accordingto claim 22, wherein the zeolite is silicalite S-1. 24) A process forpreparing MFI/MEL or MEL zeolites having the formula x TiO₂·(1−x) SiO₂wherein x has a value ranging from 0.0005 to 0.03, which consists insubjecting to hydrothermal treatment at autogenous pressure, at atemperature ranging from 190 to 230° C. and for a time ranging from 0.5to 10 hours, without alkaline metals, a mixture containing a siliconsource, a titanium source, tetra-alkylammoniumhydroxide (TAAOH), havingthe following composition expressed as molar ratios: Si/Ti=35-2000TAA-OH/Si=0.2-0.5 H₂O/Si=10-35. 25) Zeolitic catalysts comprisingzeolite and oligomeric silica, in the form of microspheres, wherein thezeolite is a zeolite consisting of silicon and aluminum oxides belongingto the MFI, MFI/MEL, MEL, BEA, MOR, FAU and FAU/EMT groups preparedaccording to the process of claim
 7. 26) Zeolitic catalyst in the formof microspheres having a diameter ranging from 5 to 300 μm, consistingof silicalite and oligomeric silica, in a weight ratio oligomericsilica/silicalite ranging from 0.05 to 0.3. 27) Zeolitic catalysts inthe form of microspheres having a diameter ranging from 5 to 300 μm,consisting of oligomeric silica and a zeolite selected from TS-2 andTS-1/TS-2, in a weight ratio oligomeric silica/zeolite ranging from 0.05to 0.3.